Neurofibrillary tangles are aggregates of the protein tau a normal neuronal cytoskeletal protein that becomes highly phosphorylated and ultimately forms fibrils within neuronal soma. As Alzheimer?s disease (AD) progresses the number of tangles increases and their location spreads throughout the brain. One idea for the spread is the propagation hypothesis which suggests that spread of pathology is due to the presence in the extracellular space of misfolded tau its uptake into other neurons and aggregation of nave endogenous tau into the misfolded conformation which in a prion-like way leads to neurotoxic consequences in the downstream neurons. This hypothesis underlies current human clinical trials of anti-tau immunotherapy which are designed to block tau propagation. The current proposal explores and tests critical aspects of this model. The form of tau that might be taken up is uncertain ? our own previous work highlights a high molecular weight phosphorylated soluble species that is only ~1% of the soluble tau in an Alzheimer brain but which can be taken up by neurons in culture or in vivo and lead to aggregation. Other reports highlight other species including a sarkosyl preparation of sonicated fibrils that can also be taken up. We will study which if either of these types of post translationally modified tau are bioactive using cellular animal model and human neuropathological materials. The proposed studies will evaluate the exact biochemical characteristics of the tau species that is competent to be taken up by neurons (aim 1) the kinetics of aggregation once taken up (aim 2) and determine whether there are any marks of toxicity of propagated aggregated tau in recipient neurons both in animal models and in human neuropathological specimens (aim 3). Advanced microscopy methods including in vitro time lapse photography and image analysis to determine rates of uptake/aggregation in neurons and IPS cells in culture and longitudinal in vivo intravital multiphoton microscopy of intact living mice exposed to various tau preparations will determine the kinetics and efficiency of uptake and aggregation of the different preparations of tau derived from human brain; a set of morphological readouts evaluating subcellular aspects of tau toxicity will be employed in these experimental systems to assess whether uptake and aggregation of tau ?matters? ? ie what are the consequences of propagation? Those results will be compared to identically prepared samples from human Alzheimer cases testing the idea that uptake and aggregation of exogenous tau leads to the type of neuronal changes that actually occur in the human disease. The results of the proposed study will thus test the underlying assumptions of the ?propagation hypothesis? define the molecular characteristics of the propagating specie(s) and thereby inform interpretation of ongoing clinical trials and the design of future trials for AD